In a typical MOS device, a channel region is formed by doping the semiconductor substrate with impurities. The channel region may have both majority carriers and ionized impurities. The majority carriers may be scattered by the ionized impurities during a migration process. The mobility of carriers and the device performance can be reduced. As dimensions of semiconductor devices continue to shrink, short-channel effect may seriously affect device performance.
High electron mobility transistors (HEMTs) have a channel region formed by a hetero-junction. The hetero-junction is formed by an undoped quantum well (QW) layer and a barrier layer on the undoped QW layer. Two-dimensional electron gas may be formed in the QW layer and may be migrated in a horizontal plane. The two-dimensional electron gas is used as carriers of the transistor. Because the QW layer is undoped, the two-dimensional electron gas is not affected by the scattering of the ionized impurities, which provides high mobility to the carriers. Device performance is effectively improved with reduced short channel effects and reduced threshold voltage.
For HEMTs, however, the gate may have less or weak control on the extended area of the channel region under the sidewall spacer of the gate, which may result in a non-uniform density of the two-dimensional electron gas in the channel region with high resistance in the channel region and with reduced device performance.